1. Field of the Invention
The present invention generally relates to a packaging machine which is automatically controlled to perform a wrapping operation. More particularly, the present invention relates to improvements in a control device and a method for controlling a driving system of the packaging machine.
2. Description of the Prior Art
In order to produce a wrapped article by using, for example, a horizontal pillow-type packaging machine, a web of continuous film is led from a film stocker containing a rolled film and is then formed into a tube during its travel. After articles are filled into the film tube at predetermined intervals, the film tube is sealed transversely with respect to the film running direction between the adjacent articles and optionally is cut at the sealed portions. The film web is often printed with ornamental designs, characters, pictures and the like at regular intervals.
The above packing operation requires a coordination among the feeding position of the article to be packed, the cutting and sealing position of the film between each article, and the leading timing of the film web so that the print is arranged at a correct position on the wrapped article.
A typical packaging machine employs a single driving motor from which driving power is transmitted to all driven units through various transmissions such as cam and gear mechanisms, but the machine should be pre-adjusted to perform the above positioning operation in a complicated manner prior to its operation. More specifically, each of the transmissions should be released from the motor so that a position of each driven unit can be temporarily adjusted. Then the driven unit is connected to the motor and subjected to test running to check whether the machine properly works or not. If the positions are not coordinated, the transmission should be released from the motor again to re-adjust the driven units. Thus, the initial setting of the machine is a time-consuming and troublesome process.
Recently, a packaging machine has been developed which includes a plurality of driving motors for respective driven units. Since these driving motors can be independently operated, the initial adjustment of the driven units can be easily and correctly performed. In such a multi-motor machine, a control operation is essentially required to synchronize the motors to each other so that the normal wrapping operation subsequent to the initial adjustment may be carried out smoothly and accurately. A typical example of such a control operation is disclosed in Japanese Pat. KOKAI document No. 61-259927 in which motors for the film leading unit and for the sealing and cutting unit, respectively, are controlled with a motor for the feed unit being a reference. Specifically, the feed unit is driven at a constant running speed, while the film traveling speed is controlled through a detection of register marks at which the film is to be cut and the cutting and sealing unit is also controlled to vary its rotational speed as well as its engaging timing.
In the initial adjustment, an attachment or a so-called flight that is formed on the feed unit to forward the article is stopped at any appropriate position. Because a distance between the flight and the cutting and sealing unit can be calculated, an initial position of the latter unit can also be calculated based on the calculated distance and other factors including an angle of rotation of the unit over a period of time during which the article is to travel from the flight to a position adjacent the unit. Therefore, the cutting and sealing unit may be correctly positioned relative to the feed unit. On the other hand, the register marks on the film web are detected by a sensor which sends to a control unit a signal for stopping the travel of the film web so that the register mark stops at a reference detection location of the sensor. The sensor is stationarily fixed adjacent the film stocker and a distance between the sensor and the cutting and sealing unit is constant. Thus, the initial position of the film web determined by the sensor does not always coincide with the initial position of the cutting and sealing unit which has been determined to coordinate the same with the feed unit, resulting in a difficulty in the initial adjustment of the film traveling unit.
During the subsequent wrapping operation, the feed unit runs at a constant speed as mentioned above. If an interval between adjacent register marks, which is detected by the sensor, is different from a preset or input value due to a printing error or an elongation of the film itself, the film traveling speed as well as the rotational speed of the cutting and sealing unit is varied so that each article may be packed at the correct position. However, such variation in the film speed which is caused each time the sensor detects the interval difference renders the film tension unstable, causing the film to fluctuate during its travel and preventing the cutting and sealing unit from functioning as desired. Further, the film fluctuation or waving continues after the film is formed into the tube into which the articles are supplied. This tends to displace the articles relative to the film tube that is to be cut at regular intervals. Additionally, the peripheral speed of the cutting and sealing unit must coincide with the film speed while they are in contact with each other. Thus, the peripheral speed should be varied whenever the film speed changes. However, such a control is very complicated since there is no direct association between the controllers of the motor for the cutting and sealing unit and the motor for the film traveling unit.